Some sheet feed mechanisms mounted on printers use gear-shaped sheet pressing wheels (hereinafter referred to as "star wheels") in which tooth portions with comparatively sharp tips are arrayed in an outer circumferential portion of each star wheel. A plurality of such star wheels are attached rotatably around a shaft disposed perpendicularly to the sheet feed direction, and each star wheel is disposed in opposition to a paper discharge roller. The star wheels catch a sheet, which is fed in a predetermined feed course by drive rollers, by means of the comparatively sharp tips of the tooth portions in their circumferential portions while the sheet is held between the sheet discharge rollers and the star wheels, so as to prevent the sheet from slipping off to the side and control the sheet to run in a regular feed direction.
Conventionally, when such a star wheel is manufactured, a gear-like plane shape is first formed by a punching process, and tips of tooth portions are thereafter thinned by an etching process in order to form acuminated shapes in the tips of the tooth portions. This is because the curvature radius in the plane shape of the tip of each tooth portion of the star wheel has to be made extremely small as 0.04 mm, and the thickness of the tip of the tooth portion has to be also made extremely small as 0.06 mm.
If the area of the tip of the tooth portion becomes large in the star wheel, in the case where ink printed on a sheet adheres to the tip of the tooth portion, this ink is transferred to other portions of the sheet so that spotted marks large enough to be visible are left on the sheet. On the other hand, if the tip portion of the tooth portion of the star wheel is too sharp or has a burr, the tip portion may damage the surface of the sheet discharge roller, or cut into the sheet and damage it. Therefore, the shape of the tip of any tooth portion of the star wheel has a very fine shape as mentioned above, and manufacturing precision is demanded.
In the above-mentioned manufacturing method, however, process control and others take a great deal of labor in the etching process so as to increase the cost. In addition, the corners of the tip portions get pointed by the etching process, so that there is a problem that it is impossible to avoid the above-mentioned disadvantage.
In addition, in the case where the plane shape is formed by press-working as mentioned above, there is a problem that since the curvature radius of the tip of each tooth portion is so small that it is difficult to perform press-working on a thick metal sheet, it is difficult to ensure the rigidity of the star wheel. Further, since very fine shapes having a small curvature radius have to be formed, there is another problem that a load on a press die becomes so large as to shorten the life of the press die.
Further, since there is an etching process after press-working as mentioned above, it is necessary to carry very small and individually separated star wheels and set them for the etching process. Such working is extremely troublesome, and it is difficult to mechanize all the working. Therefore, there is a problem that increase of the manufacturing cost is caused and the manufacturing efficiency cannot be improved to be higher than the current one.